Multi-lingual web application – how do I detect the user’s language in ASP.NET?

I’m building an ASP.NET web application, and all of my strings are stored in a resource file. I’d like to add a second language to my application, and ideally, I’d like to auto-detect the user’s browser language (or windows language) and default to that, instead of making them choose something besides English. Currently, I’m handling all the resource population manually, so adding a second resource file and language is trivial from my point of view, if I had an easy way to automatically figure out what language to display.